I was complaining about "conservative" and "liberal" in American politics having nothing to do with their actual traditional definitions before I ever cracked an Orwell book. The false notion that there's a huge gulf between "the right" and "the left" is how they manage to cling to power. (In reality, they're more like "the right" and "the center-right", with a few center-left in there and I guess Bernie Sanders off to the actual left somewhere. Inasmuch as a single axis can accurately reflect the complexity of political thought, which...well, it can't in real life but it's probably pretty close as far as American electoral politics are concerned, more's the pity.)
My favorite linguistic example of how fucked our political language has become is that the people we call "conservatives" are generally the people who are opposed to conservation. Never mind that things like wars of choice and eliminating all federal government funding for anything except wars of choice are pretty fucking radical ideas and not traditionally conservative in the least.
But yes, to the point of the fundamental difference between Limbaugh and Maher: you've nailed it. Democrats are not afraid of Bill Maher. I don't think it's quite accurate to say that Rush is running the Republican Party or that it's taking its cues from him (though you could easily make that argument for, say, Norquist), but he DOES have the power to seriously fucking inconvenience any Republican who criticizes him.
This is part of the same usual stuff we gripe about, I think: the liberal base is less monolithic, and votes in smaller numbers, than the conservative (granting our caveats about how fucking inappropriate those words actually are). Fewer people listen to Bill Maher than Rush Limbaugh, and of those not that damn many are going to vote based on what Bill Maher says. (And indeed even Bill Maher's fans probably have an easier time criticizing him than Rush's fans have criticizing him. I liked Religulous and I like a lot of what Maher has to say, I think he usually has pretty good guests and often has good debate topics. But I also think the guy's an ass, and a misogynist besides, and that that's why he's backing Rush up in this case, because the one topic that unites the two of them is that they both like saying terrible things about women.)
(Also, Maher's fans KNOW he's a comedian.)